1. Field of the Invention
The present invention relates to an optical disk, a method for producing the same, and a recording and reproducing apparatus. In particular, the present invention relates to an optical disk which stably reproduces information recorded on optical disks having different reflectances, a method for producing the same, and a recording and reproducing apparatus.
2. Description of the Related Art
DVD, which has appeared in recent years, has a lineup including, for example, not only information read-only DVD-ROM and DVD-Video but also information recordable DVD-R and information rewritable DVD-RAM and DVD-RW. Therefore, the DVD market is rapidly expanded not only as the screen image-recording and reproducing medium which substitutes VTR but also as the large capacity medium for computers. Recently, the digital broadcast has come into widespread use, which simultaneously increases the demand for any medium capable of responding to the high detailed image and the long time recording and the demand for any large capacity storage medium capable of responding to the digital data increasing day after day.
A variety of techniques have been hitherto suggested in the field of the optical disk in order to realize the high density. Those having been suggested include, for example, a method in which the recording mark is made fine and minute by using the blue laser having a shorter wavelength (λ=405 nm) and a method in which the high density is realized by recording information on both of the land and the groove of the optical disk. In view of the format, various optical disks have been also suggested, in which not only the data-recording section but also the header section structure to store, for example, the address information is contrived. For example, in the case of iD-photo, the guide groove is deflected in the radial direction of the track to record information about the header section on only one side of the recording track, and thus the format efficiency is enhanced without longly cutting the recording track.
The phase-change recording system, which is adopted, for example, for DVD-RAM and DVD-RW, is generally used in relation to the technique of the information rewritable optical disk. In the case of the optical disk based on the phase-change recording system, a phase-change material is used for a recording layer. Basically, pieces of information of “0” and “1” are recorded to correspond to the crystalline state (non-recorded state) and the amorphous state (recorded state) of the phase-change material respectively. The refractive index differs between areas in the crystalline state and the amorphous state formed in the recording layer. Therefore, for example, the refractive indexes and the thicknesses of the respective layers for constructing the optical disk are designed so that the difference in reflectance is maximized between the portion changed into the crystalline state and the portion changed into the amorphous state. In the case of the optical disk based on the phase-change recording system, the light beam is radiated onto the crystallized portion and the amorphous portion to detect the difference in the reflected light amount between the respective portions of the optical disk so that “0” and “1”, which are recorded in the recording layer, are detected.
In order that the amorphous state is realized at a predetermined position of the recording layer of the optical disk based on the phase-change recording system (usually, this operation is referred to as “recording”), the light beam having a relatively high power is radiated so that the temperature of the irradiated portion of the recording layer is raised to a temperature which is not less than the melting point of the recording layer material. On the other hand, in order to effect the crystallization at a predetermined position of the recording layer (usually, this operation is referred to as “erasing”), the light beam having a relatively low power is radiated so that the temperature of the irradiated portion of the recording layer is raised to a temperature which is not more than the melting point of the recording layer material and which is in the vicinity of the crystallization temperature. As described above, in the case of the optical disk based on the phase-change recording system, the predetermined portion in the recording layer can be reversibly changed between the amorphous state and the crystalline state by regulating the radiation power of the light beam to be radiated onto the recording layer.
In the case of the conventional optical disk based on the phase-change recording system, it has been tried to improve the various characteristics of the optical disk by not only increasing the difference in the reflectance between the amorphous state and the crystalline state in the recording layer but also by adjusting the relationship of relative magnitude between the reflectances of the both states. For example, in the case of ordinary DVD-RAM and DVD-RW, the disk is designed so that the reflectance is high at the portion in the crystalline state corresponding to the non-recorded state, and the reflectance is low at the portion in the amorphous state corresponding to the recorded state. Therefore, in the case of the optical disk as described above, the following advantage is obtained, because the address information is recorded in the area in the non-recorded state. That is, the address information is obtained at high S/N, and the reliability of the address information reproduction is improved. Further, in the case of the optical disk as described above, the following advantage is obtained, because the focus servo signal and the tracking servo signal are also obtained from the area in the non-recorded state. That is, it is possible to obtain the tracking servo signal and the focus servo signal having the sufficient intensities, and it is possible to perform, for example, the tracking control in a stable manner.
On the other hand, in relation to the technique of the write-once optical disk capable of recording information only once, a method is generally used, in which a recording layer is formed by using a material containing an organic dye as adopted, for example, for DVD-R. When information is recorded in the recording layer containing the organic dye, the recording mark is formed in the recording layer by causing, for example, at least one of the change of the optical parameter of the organic dye in the recording layer, the deformation of the recording layer as well as the reflective layer and the substrate disposed close to the recording layer, and the gap generated in the recording layer or at the recording layer interface by radiating the light beam having a relatively high power. In the case of DVD-R, the setting is made such that the reflectance is high in the non-recorded state and the reflectance is low in the recorded state, in the same manner as in DVD-RAM and DVD-RW. Therefore, DVD-R is advantageous in that the address information is obtained at high S/N, and the reliability is improved when the address information is reproduced. Further, the focus servo signal and the tracking servo signal are also obtained from the area in the non-recorded state. Therefore, DVD-R is advantageous in that it is possible to obtain the tracking servo signal and the focus servo signal having the sufficient intensities, and it is possible to perform, for example, the control of the tracking in a stable manner.
In this specification, the optical disk, which has the following feature like the conventional DVD-RAM, DVD-RW, and DVD-R, is referred to as “High to Low” disk or simply “HL” disk. That is, when information is recorded by radiating the light beam, the reflectance of the portion irradiated with the light beam (portion in the recorded state) is changed from the high value (high level) to the low value (low level). In general, the HL disk based on the phase-change recording system has a structure including at least a protective layer composed of a transmissive dielectric, a phase-change recording layer, an intermediate layer composed of a transmissive dielectric, and a heat-diffusing layer composed of metal (having a function of a reflective layer as well) which are disposed in this order from the light beam incoming side. Further, the HL disk, which is based on the use of a recording layer containing an organic dye, generally has a structure including at least a recording layer containing an organic dye and a heat-diffusing layer composed of metal (having a function of a reflective layer as well) which are disposed in this order from the light beam incoming side.
In relation to the conventional optical disk based on the phase-change recording system, an optical disk has been also suggested, which is designed such that the reflectance is low in the area in the crystalline state corresponding to the non-recorded state in the recording layer of the optical disk, and the reflectance is high in the area in the amorphous state corresponding to the recorded state (see, for example, Japanese Patent No. 2512087, pp. 4-6, FIG. 1). The optical disk, which is disclosed in Japanese Patent No. 2512087, is advantageous in that old information is erased at a larger ratio when new information is overwritten on the old information.
On the other hand, the optical disk (hereinafter referred to as “organic dye type optical disk”), which is based on the use of the recording layer containing the organic dye, can be also designed such that the reflectance is low in the area in the non-recorded area, and the reflectance is high in the area corresponding to the recorded state. Specifically, the recording mark is formed by causing the change of the optical parameter of the organic dye in the recording layer, while extremely avoiding the deformation of the recording layer as well as the substrate and the reflective layer and the gap generated in the recording layer or at the recording layer interface when the recording mark is formed as described above. Accordingly, the reflectance can be made low in the area in the non-recorded state, and the reflectance can be made high in the area corresponding to the recorded state. The optical disk as described above is advantageous in that the recording sensitivity is improved, because the reflectance is low in the non-recorded area.
In this specification, the optical disk, which has the following feature like the optical disk disclosed in Japanese Patent No. 2512087, is referred to as “Low to High” disk or simply “LH” disk. That is, when information is recorded by radiating the light beam, the reflectance of the portion irradiated with the light beam (portion in the recorded state) is changed from the low value (low level) to the high value (high level). The LH disk, which is based on the phase-change recording system, has a structure as represented by multilayer films including, for example, a recording layer/dielectric layer, a recording layer/metal reflective layer, and a dielectric layer/recording layer/dielectric layer/metal reflective layer. The interference effect is utilized between the films for constructing the multilayer film. Accordingly, the reflectance is low in the area in the crystalline state corresponding to the non-recorded state, and the reflectance is high in the area in the amorphous state corresponding to the recorded state.
Conventionally, an LH disk based on the phase-change recording system has been also suggested (see, for example, Proc. SPIE, vol. 3401, p. 103, 1998), which has a multilayer film structure comprising ZnS—SiO2/SiO2/ZnS—SiO2/Ge2Sb2Te5/ZnS—SiO2/Al alloy. Also in the case of the LH disk, the interference effect is utilized between the films for constructing the multilayer film so that the reflectance is low in the area in the crystalline state corresponding to the non-recorded state, and the reflectance is high in the area in the amorphous state corresponding to the recorded state.
As described above, both of the HL disk and the LH disk have been hitherto investigated in the field of the optical disk based on the phase-change recording system and in the field of the organic dye type optical disk. When each of the optical disks is installed to a conventional recording and reproducing apparatus to record and reproduce information, the following problems arise due to the difference in the film structure. (1) The reflectance in the non-recorded state (crystalline state in the case of the phase-change type) differs between the HL disk and the LH disk. (2) The relationship between the reflectance in the non-recorded state and the average reflectance after the information recording differs between the HL disk and the LH disk. That is, the values of the reflectances in the areas in the non-recorded state and the recorded state and the relationship of relative magnitude therebetween differ depending on whether the optical disk is either the HL disk or the LH disk. Therefore, the level of the reproduced signal to be obtained differs as well. Therefore, a problem arises such that it is impossible to reproduce information stably and highly reliably from both of the HL disk and the LH disk by using any identical recording and reproducing apparatus.
For example, the following method may be conceived in order to solve the problem as described above. That is, any information, which indicates that the optical disk installed to the recording and reproducing apparatus is either the HL disk or the LH disk (hereinafter referred to as “type of the optical disk” as well), is recorded in a control data area for storing the physical format information about the optical disk. The gain is optimally adjusted on the basis of the information, and then the user information or the like is reproduced.
In general, the control data of the optical disk is formed by emboss pits. The control data area and the user data area are formed approximately adjacently to one another. Therefore, in the case of the optical disk based on the phase-change recording system and the optical disk of the organic dye type, for example, the phase-change film, the protective film, and the organic dye film are formed not only in the user data area but also in the control data area respectively. In the case of the conventional optical disks as described above, even when the information concerning the type of the optical disk is recorded in the control data area, the reflectance of the control data area is also changed depending on the type of the optical disk. Therefore, if the gain is not adjusted, it is impossible to accurately reproduce not only the control data but also the information concerning the type of the optical disk. Therefore, in the case of the optical disk on which the information concerning the type of the optical disk is recorded in the control data area, it is necessary that the information concerning the type of the optical disk and the control data are reproduced while adjusting the gain of the reproduced signal depending on the type of the optical disk. Therefore, a problem arises such that the gain adjustment requires a long period of time, and it is impossible to quickly reproduce the control data and the user data. However, at present, there is no method for solving this problem. It is also impossible to find any method for optimizing the gain depending on the type of the optical disk (HL disk or LH disk).